I recently had a gluten free cream tea. On eating a ham sandwich I realised it had a smear of English mustard on it, which is not gf! The rest was delicious.
I had a discreet word with the chef on leaving, and mentioned this to him. He was completely unaware of this issue and was going to do more research. Fortunately I was not badly affected as I did not have much, but I am so careful and seldom eat out.
It was nice to be able to enjoy a treat and also assist a well meaning chef increase his knowledge.